I'm not sure which forum this should go into, so I'll put it here.
Observe please, the color of the smoke boxes of the Lionel 2-8-0 and the American Flyer 312:
When I got the 312 (probably about 8 years ago) it was more or less a project engine, so I didn't mind painting the smoke box a different color. I had used this paint before, and really like it. It's kind of expensive, but is easily found and works great, I think:
This brand comes in many different colors; I like this one for highlighting fireboxes, smokeboxes, etc. #6201 "Charcoal".
Although it is made for stoves, if a primer is used first I have been able to spray this on plastics as well.
Just an FYI...
Mark in Oregon